Titanium aluminide intermetallic matrix composite (TA-IMC) materials offer exceptional properties compared to conventional alloys and other composite materials. TA-IMC materials have low density (3.4-3.7 g/cc), high elastic modulus (170-210 GPa), high wear resistance, and operational temperatures as high as 900° C. Compared to conventional steel and aluminum alloys, TA-IMC materials offer greater specific strength and specific elastic modulus. Compared to conventional continuous fiber composites, such as ceramic fiber reinforced aluminum and carbon or glass fiber reinforced polymeric materials, TA-IMC materials offer substantially greater ductility and excellent transverse properties due to their isotropic nature. TA-IMC materials also offer a significantly higher operating temperature compared to these other conventional materials, and are not susceptible to the environmental problems associated with polymeric composites, such as corrosion, degradation and delamination as a result of exposure to moisture, heat and ultraviolet radiation.
An intermetallic is a metal alloy where the composition of at least two constituent metals is considered to be middle range, resulting in a solid phase crystalline material formed by an ordered structure of the two metal atom types. The most common titanium aluminide intermetallic solid phases are TiAl, TiAl3, and Ti3Al, with the preferred phase being TiAl due to its superior mechanical properties. Depending on the composition, a predominately TiAl intermetallic may also contain trace amounts of TiAl3 and Ti3Al. The TiAl intermetallic phase is often identified by the Greek letter γ (gamma). The phases where titanium is approximately 20-80% of the composition by weight are considered middle range, with compositions of 59-65% titanium by weight being most preferred. A titanium aluminide intermetallic composite material consists of a titanium aluminide intermetallic matrix, reinforced by some other material, usually a ceramic or metal oxide such as aluminum oxide (Al2O3, alumina). Reinforcement materials can be in the form of particles, short fibers or whiskers, or continuous fibers. Titanium aluminide intermetallic composite materials containing in situ formed alumina particles can be produced by the combustion reaction of aluminum (Al) and titanium dioxide (TiO2, titania) to yield TiAl and alumina. The combustion synthesis reaction between aluminum and titania is known to be initiated at a temperature greater than 850° C.
Despite their numerous advantages, known TA-IMC materials suffer drawbacks that have hampered their use in many engineering applications. In fully dense form, the mechanical and physical properties of the bulk TA-IMC material are exceptional; however, due to crystal densification resulting from the transformation of aluminum and titania into titanium aluminide and alumina during the combustion synthesis reaction, a substantial amount of void content, or porosity, is created. The resulting void content has a significant adverse effect on the mechanical and physical properties of the TA-IMC material, rendering it unusable in this state for practical engineering applications. A known approach for eliminating porosity in combustion synthesized TA-IMC materials is to manufacture a ceramic preform containing titania particles combined with particles of an alkali metal titanate, such as lithium titanate of the chemical form Li2TiO3. The rigid and porous ceramic preform is then infiltrated with molten aluminum to form a pre-combustion material. During the subsequent combustion synthesis reaction which occurs spontaneously at a temperature well above the melting temperature of aluminum, the lithium titanate is chemically reduced by the molten aluminum to form lithium aluminate of the chemical form LiAlO2. This process results in a volumetric expansion caused by the lower density of lithium aluminate compared to that of lithium titanate, which in turn counteracts the densification of the titanium aluminide to an extent sufficient to eliminate void formation during combustion synthesis. Alkali metals such as lithium are known however to be highly corrosive, and TA-IMC materials containing alkali metals cannot withstand high electrical voltage, high strain and high temperature conditions associated with electrical power transmission cables. Furthermore, the process of producing TA-IMC materials by means of a pre-combustion material comprising a rigid and porous ceramic preform is entirely unsuitable for the continuous manufacture of a wire.
While titanium aluminide intermetallic alloys are known, these materials are cost prohibitive with regards to producing a wire for electric power transmission cables due to the high cost of titanium metal and the metallurgical processes required to produce the alloy. In contrast, combustion synthesized TA-IMC materials are produced using a low energy, low cost process and utilize low cost raw materials in the form of aluminum metal and titania.
In view of the above, a need exists for fully-dense TA-IMC materials produced using a low cost process and low cost raw materials, but without the use of rigid ceramic preforms or alkali metal titanates, and which exhibit excellent mechanical and physical properties under high electric voltage, high strain and high temperature conditions. In particular, a need exists for a TA-IMC wire for electrical power transmission cables that are free from long term corrosion and degradation problems under loading conditions, and impervious to adverse environmental elements such as moisture and ultraviolet radiation.
The present invention pertains to a wire of combustion synthesized TA-IMC material. A preferred embodiment of the present invention pertains to the continuous combustion synthesis of TA-IMC from a pre-combustion feedstock material comprising elemental aluminum and titanium oxide (titania) followed by thermo-mechanical forming to eliminate the porosity inherently found in combustion synthesized TA-IMC material, and thereby forming a fully dense TA-IMC wire. The feedstock material, comprising elemental aluminum and titania particles, is itself in the form of a wire which may be produced by conventional means. The titania particles of the feedstock material may be of the chemical composition TiO, TiO2, Ti2O3 or any combination thereof.
In the present invention the feedstock material is continuously fed into an enclosed chamber or reactor which contains a heating means to sufficiently heat a section of the continuously fed feedstock as to initiate the Ti—Al combustion synthesis reaction. The speed of the feeding mechanism is maintained such that the combustion front within the feedstock material remains enclosed within the confines of the reactor. Because the Ti—Al synthesis reaction is exothermic, additional heat need only be applied as necessary to continuously maintain the combustion reaction. The reactor chamber may contain an atmosphere of air or inert gas, or a vacuum may be applied around the feedstock wire at the point of combustion. As the combustion synthesized TA-IMC wire exits the reactor, additional heat may be applied as necessary to maintain a desired temperature optimal for thermo-mechanical forming.
Upon exiting the reactor chamber, the hot TA-IMC wire is drawn through one or more wire forming dies such that its diameter is sufficiently reduced as to eliminate void content, impart axial elongation of the Ti—Al grain structure and uniformly orient in situ formed alumina particles, thereby achieving the desired mechanical properties along the continuous length of the wire. At temperatures above 1150° C., the gamma (γ) phase titanium aluminide will partially transform into the alpha (α) phase titanium aluminide, and possibly some metastable beta (β) phase titanium aluminide, both of which increase the hot-workability of the material. Furthermore, the relative abundance of α and β phases present at the optimum thermo-mechanical processing temperature can be increased by adding various alloying elements to the pre-combustion feedstock such that in the post-combustion synthesized intermetallic alloy these elements are less than 5% by weight. These alloying elements include vanadium (V), niobium (Nb), molybdenum (Mo), and Boron (B). Finally, the present invention pertains to a plurality of said wires such as to form the reinforcing core of an assembled electric power transmission cable.
TA-IMC wires of the present invention are useful in numerous applications. Such wires are particularly desirable for use in electric power transmission cables due to their combination of low weight, high strength, high elastic modulus, good electrical conductivity, low coefficient of thermal expansion, high operating temperatures, resistance to corrosion and high ductility. The technical benefit and overall utility of TA-IMC wires of the present invention for use in electric power transmission cables, is a result of the significant effect cable performance has on the entire electricity generation, transmission and distribution system.
The design of an electric power transmission system consists primarily of power transmission cables and supporting structures. The load bearing capacity required of the supporting structure is determined by the density of the cables, the number of cables, and length, or span, of the cables. Specifically, the span is the linear distance between two adjacent structures connected by the cables. For a given electric power transmission system design of specified voltage and amperage, power transmission cables comprising TA-IMC wires have a lower density compared to conventional cables comprising a core of steel wires. Further, the lower thermal expansion of cables comprising TA-IMC wires compared to conventional cables comprising steel wires results in less cable sag at a given operating temperature. In the design of the supporting structures, lower density cables enable the use of lower load capacity structures, and the lower degree of sag enables the use of structures of lower height, both of which reduce the cost of structures, thereby providing great economic benefit to the overall electric power transmission system.
Electrical power transmission cables of the present invention, having higher strength per unit weight, combined with increased conductivity, lower thermal expansion and high ductility provide the ability to install longer cable spans than are possible with conventional steel or composite fiber cable, and cable supporting towers of lower height and lower mechanical load capacity are also possible. Further, the high ductility of TA-IMC wires according to the present invention enables the use of standard installation tools and splices, and avoids the catastrophic brittle failure of the reinforcing core which is known to occur with continuous fiber type composite material cables. Still further, the high electrical conductivity and low electrical resistivity of the TA-IMC wire of the present invention improves the electrical properties and performance of the conductor cable and serves to reduce electrical losses, thereby minimizing the need for additional electric power generation to compensate for such losses.
When compared to other low-density electric power transmission cables known to the art, primarily cables comprising a core of continuous fiber composite type wires, cables of the present invention comprising a TA-IMC wire core offer additional advantages. Primarily, continuous fiber composite type wires exhibit no ductility along the longitudinal direction of the wire and are therefore known to be susceptible to sudden, catastrophic failure. Unlike continuous fiber type composite materials, TA-IMC materials are generally isotropic and exhibit ductility and strength in all directions. The grain elongation of the TA-IMC materials that occurs during the thermo-mechanical wire drawing process of the present invention serves to maximize the strength of the material in the longitudinal direction of the wire. Because of the isotropic nature and high ductility of TA-IMC wires, electric power transmission cables comprising a core of such wires may be spliced and installed using the same standard tools as are used with cables comprising a core of steel wires.
From the foregoing disclosure and the following more detailed description of the preferred embodiment of the present invention it will be apparent to those skilled in the art that the present invention provides a significant advance in the technology of intermetallic composite wire and electric power transmission cable. Particularly significant in this regard is the potential the invention affords for providing light weight electric power transmission cables capable of operating at higher temperatures compared to conventional electric power transmission cables reinforced with steel wires due to the low density, high strength, high elastic modulus and low coefficient of thermal expansion of the TA-IMC wire. It will be further apparent to those skilled in the art that the present invention provides a significant advantage due to the high ductility, durability and the resistance to corrosion and environmental degradation of the wire compared to other composite materials. Additional features and advantages of various preferred embodiments will be better understood in view of the detailed description provided below.